If more water leaves the body than enters it, dehydration may occur. This might, for example, occur with vigorous exercising or when someone has diarrhea.
Excessive urination can also lead to dehydration very quickly. Some of the most common triggers of excessive urination are taking diuretic medications (“water pills”), drinking alcohol, and having certain health conditions, such as high blood sugar (hyperglycemia, a condition seen in untreated diabetes).
Dehydration is also more likely to occur in warmer climates, at higher altitudes, and when someone has a fever.